The present invention relates to a rubber composition or, more particularly, to a rubber composition based on an ethylene-propylene copolymeric rubber curable into an elastomer having excellent properties in respect of heat resistance, permanent compression set and electric insulation as well as receptivity of coloring agents other than carbon black and useful, for example, as a material of parts in automobiles and other machines including rubber rollers, gaskets, packings, rubber hoses, plug boots, terminal caps and the like.
As is well known, rubbery copolymers of ethylene and propylene such as the so-called EPM and EPDM rubbers, referred to as EP rubbers hereinbelow, have excellent mechanical strengths and electric properties so that they are widely used as a material in various machines including automobiles. Disadvantages in these EP rubbers, however, are that the color of the vulcanizates of EP rubbers is always black because the reinforcing filler used in the rubber composition thereof is limited to carbon black and, accordingly, the electric insulation of the vulcanizates cannot be high enough.
A proposal of course has been made to compound an EP rubber-based composition, in place of carbon black, with a light-colored reinforcing filler such as a finely divided silica filler. The silica-filled EP rubber composition has a light color as a matter of course and, in addition, the vulcanizates thereof are imparted with improved electric insulation. A problem in the use of such a silica filler is that the silica particles inherently have silanolic hydroxy groups, i.e. --Si--OH, on the surface and in the pores while the silanolic groups having acidic nature sometimes cause inhibition of vulcanization using an organic peroxide as the vulcanizing agent along with some adverse influences on the heat resistance and permanent compression set or recovery from compression of the vulcanizates.
Another proposal has been made in Japanese Patent Publication No. 57-17011 with an object to solve the above described problems to give a rubber composition by compounding natural rubber or an organic synthetic rubber, e.g. EP rubber, with a siliceous reinforcing filler, an organopolysiloxane having aliphatically unsaturated groups and mercapto groups and an organosilicon compound, e.g. organosilane compound, having, in a molecule, at least one hydroxy or alkoxy group directly bonded to the silicon atom and a polysulfide linkage in the organic group bonded to the silicon atom. Such a formulation of the rubber composition is indeed effective to some extent to improve the mechanical strengths and heat resistance of the vulcanizate while little improvement is obtained in respect of the permanent compression set.